This invention relates to improvements in a microstrip circularly-polarized antenna array and more particularly to such an array for receiving and converting microwave power to DC current.
There is a long-standing need for low cost, low profile conformed antenna arrays which can be mass produced. That need has been more fully satisfied by microstrip antenna techniques than by any other prior art techniques reviewed in U.S. Pat. No. 3,803,623. According to that review of the technique, circular, elliptical, or even rectangular (but preferably elliptical) antenna elements are made by etching copper clad low-loss printed circuit board material, and a feedpoint is selected to match a low transmission line characteristic impedance, typically 50 ohms. A problem with laying out an array of antenna elements is providing the proper feedpoint for the desired input impedance in elements of a given geometry. It has been discovered that this problem is greatly alleviated by setting in the feedpoint sufficiently to reduce the input impedance to the value required for whatever feedpoint is selected.
When an array of dipole elements is employed to convert microwave energy for power, as for propulsion of a vehicle, it is desirable for the energy received to be rectified at the antenna array. Such an arrangement is commonly referred to as a "rectenna" because it combines the function of rectification with that of receiving microwave energy in the antenna array by placing rectifying diodes between dipole elements to break up the large aperture of the array into the smaller apertures of the dipoles as disclosed in U.S. Pat. No. 3,434,678 in which each dipole is effectively terminated by a diode whose DC output feeds into a common DC load either in series or in parallel with dipole elements. A problem with such a rectenna is that optimum performance of the dipoles requires a fixed orientation with respect to the polarization angle of the transmitted beam. Other examples of rectennas are to be found in U.S. Pat. Nos. 3,535,543 and 3,852,755. An example of how rectennas are to be used may be found in U.S. Pat. No. 3,114,517. It is desirable to combine with the technology of rectennas the technology of microstrip antennas of which U.S. Pat. Nos. 3,803,623 and 3,921,177 are exemplars, but in a thin conformal array particularly suited to mounting integrally with thin structures, such as aircraft wings, requiring minimal space.